1. Field of the Invention
This invention relates to an interferometer in which a light beam is split into two light beam by a beam splitter, and the two light beams, after being made different in optical path length, are caused to interfere with each other by means of a beam splitter. An interferometer of this type has a wide range of applications, being used for example as a means for inspecting optical components, a range finder, or an interference spectrometer.
2. Prior Art
One example of a device utilizing an interferometer of this type, namely, a double interference spectrometer is shown in FIG. 7. In FIG. 7, a light beam from a light source 1, after passing through an aperture 2, is made into a parallel light beam by a collimating reflecting mirror 3. The parallel light beam thus formed is split into two light beams 6 and 7 by means of a translucent (that is semi-reflecting) film 5 which is formed on a substrate 4 for instance by vacuum deposition. The substrate 4 is generally a parallel planar plate which transmits light sufficiently. In the case of a spectroscope for visible range, the substrate is made of glass or molten quartz. In the case of a spectroscope for infrared range, a single crystal of KBr, CsI or KRS-5 is employed to form the substrate 4. In the case of a spectroscope for far infrared range, it is unnecessary to use the substrate 4 and a correcting plate 8, and a macromolecular film of polyethyleneterephthalate or the like is employed as the translucent film 5.
The correcting plate 8 is equal both in material and in thickness to the substrate 4. The correcting plate 8 is disposed in such a manner that the translucent film 5 is located between the plate 8 and the substrate 4. For instance when the light beam is split into two light beams at the point A, the two light beams, being reflected respectively by a movable plane mirror 9 and a stationary plane mirror 10, interfere with each other at the point B. If, in this case, no correcting plate 8 is provided beside the substrate 4, the beam 7 will pass through the substrate 4 twice after the beam splitting operation, while the beam 6 will not pass through the substrate 4 at all. Since the refractive index of the substrate 4 depends on the wavelength of light, the optical distance between the stationary reflecting mirror 10 and the translucent film 5 changes with the wavelength. In the case where it is required for the two light beams to interfere with each other with a constant difference in optical path length irrespective of the wavelength, it is necessary to insert the correcting plate 8 equal both in thickness and in material to the substrate 4 in the light beam 6, thereby to provide an optical distance for the light beam 6 which depends on the wavelength similarly as in the light beam 7.
In the double beam interference spectrometer of FIG. 7, all light beams in its spectral wavelength range should undergo interference with the same difference in optical path length, and it is essential to use the correcting plate 8.
A drive device 11 is to move the movable plane mirror 9. Its movement, i.e., the difference in optical path length between the light beams 6 and 7 is detected by a laser interference length measuring machine 12 in laser wavelengths and applied to a computer 13 in real time. If the movable plane mirror 9 and the stationary plane mirror 10 are inclined even slightly, the light beam reflection direction and the optical path length difference involve errors which will considerably adversely affect the measurement. Therefore, in general, the drive device 11 (uses air bearings, and the stationary plane mirror 10 is provided with an elevation angle adjusting device so that the angle of elevation of the mirror 10 is adjusted periodically.
The light beams thus subjected to interference with the predetermined optical path length difference are applied to a specimen 15 by an illuminating mirror 14, where rays different in wavelength are absorbed according to the spectral characteristic of the specimen 15. The light beam passed through the specimen 15 and reflected by a mirror 16 is applied to a detector 18 by a detecting mirror 17. The output of the detector 18 is applied, as an electrical input, to the computer 13. In the computer 13, the detector output as a function of the optical path length difference measured with the laser interference length measuring machine is stored as data, and after the data have been stored as much as the predetermined optical path length difference, the spectral characteristic is obtained by Fourier transformation.
The substrate 4 and the translucent film 5 forming a beam splitter together with the correcting plate 8 are shown in FIG. 8 in more detail. In general, the substrate 4 is in the form of a disk, and its diameter should be about 10% of the diameter because it must be polished as described later. The substrate 4 and the correcting plate 8 are made of a transparent material. For an intermediate infrared region, they are generally made of an expensive KBr single crystal. The surfaces of the substrate 4 and the correcting plate 8 must be optically polished; however, the polishing cost is considerably high because a KBr single crystal is relatively soft and deliquescent. Furthermore, the substrate 4 and the correcting plate 8 must be equal in thickness, and in the case of a high resolution spectrometer the thickness tolerance should be of the order of .+-.10 .mu.m. Thus, in addition to the polishing operation, the operation of making the substrate and the correcting plate equal in thickness is required. This will further increase the manufacturing cost. For a visible range, the substrate and the correcting plate can be made of molten quartz; i.e., the material cost is relatively low; however, because of the short measurement wavelength they must be polished higher in flatness than those for infrared range. In addition, the difference in thickness between the substrate 4 and the correcting plate 8 must be small. Thus, the manufacturing cost of the beam splitter for visible range is substantially equal to that of the beam splitter for intermediate infrared range. That is, the beam splitter 19 makes up a large part of the manufacturing cost of the spectrometer, being 20% to 40% of the direct material cost of the spectrometer excluding the computer 13.
When compared with a grating type spectroscope, the interference spectrometer has been markedly improved in sensitivity, and extensively employed in the industry, thus becoming, an essential instrument in the high technical industry. However, the interference spectrometer is much more expensive than the grating type spectroscope. In the spectrometer body, the beam splitter 19 and the computer 13 required for Fourier transformation are main factors of the high manufacturing cost. A computer 13, less expensive but more capable, has been developed with the advancement of electronics. On the other hand, a beam splitter 19 is fundamentally the same as that which was proposed several decades ago. Therefore, there has been a strong demand for the provision of a beam splitter 19 of improved performance and decreased manufacturing cost.